The sentence has correct grammar, but the phrase "deserving to be relied upon" is not idiomatic. A more natural way to express the idea could be: "I don't think his opinion is reliable." Alternatively, you could say: "I don't think his opinion is worth relying on."

It is a bit cumbersome and also it is weak because the only active verb is "is". That verb does not convey the essential action of the sentence. Instead, the essence of the sentence is contained in the adjective "deserving". To make a strong sentence, put the action into a verb: "I don't think his opinion deserves our confidence."